1. Field of the Invention
The present invention relates to a piezoelectric resonator and a piezoelectric filter including a piezoelectric thin film, and more specifically, to a piezoelectric resonator and a piezoelectric filter in which a piezoelectric vibrating portion including a piezoelectric thin film is arranged while being separated from a substrate.
2. Description of the Related Art
Known piezoelectric resonators and piezoelectric filters include a thin piezoelectric vibrating portion including a piezoelectric thin film.
For example, Japanese Unexamined Patent Application Publication No. 2002-299980 discloses a piezoelectric filter illustrated in FIG. 37. A piezoelectric filter 501 includes recesses 502a and 502b provided on the upper surface of a support substrate 502. In a region in which the recess 502a is provided, a first piezoelectric resonator 506 including a piezoelectric thin film 503, an upper electrode 504, and a lower electrode 505 is arranged above the recess 502a. 
Similarly, a piezoelectric resonator 509 including the piezoelectric thin film 503, an upper electrode 507, and a lower electrode 508 is also arranged above the recess 502b. 
Here, the piezoelectric resonators 506 and 509 are electrically connected to each other such that the piezoelectric resonator 506 is a series arm resonator and the piezoelectric resonator 509 is a parallel arm resonator, and the piezoelectric filter 501 having a ladder circuit configuration is provided. To provide a difference between the resonant frequency of the series arm resonator and that of the parallel arm resonator, the lower electrode 505 of the piezoelectric resonator 506 and the lower electrode 508 of the other piezoelectric resonator 509 have different thicknesses.
WO99/37023 discloses an energy-trap piezoelectric resonator utilizing the thickness longitudinal vibration mode. In this piezoelectric resonator, in the portion in which an upper electrode and a lower electrode overlap each other with a piezoelectric substrate disposed therebetween, an energy-trap piezoelectric vibrating portion utilizing the thickness longitudinal vibration mode is provided. A mass adding film is disposed at the outer area of either the upper electrode or the lower electrode. The mass adding film adds mass to the surrounding area of the piezoelectric vibrating portion. This enables the thickness longitudinal vibration mode to be effectively trapped in the piezoelectric vibrating portion.
The piezoelectric filter 501 described in Japanese Unexamined Patent Application Publication No. 2002-299980 includes the piezoelectric thin film 503 having a relatively small thickness. Therefore, as compared to when a piezoelectric plate having a relatively large thickness is used, the resonant frequency of each of the piezoelectric resonators 506 and 509 can be increased. That is, the piezoelectric filter 501 capable of being used in a higher frequency range can be provided by utilizing the thickness longitudinal vibration mode.
However, a spurious component caused by another mode, such as a Lamb wave propagating in the direction of the plane of the piezoelectric thin film 503, other than the thickness longitudinal vibration mode, may occur, and favorable resonant characteristics and filter characteristics may not be obtained.